This invention relates generally to apparatuses for welding industrial textiles and, more particularly, to an apparatus for welding pieces of industrial textiles such as thermoplastic materials or thermoplastic coated fabrics together.
The industrial textile industry is based on the availability of a variety of thermoplastic extruded sheeting and thermoplastic coated fabrics. These materials are used to make a wide range of products such as, for example, inflatable boats, hot air balloons, covers for outdoor structures, geo-membranes for lining toxic waste sites, awnings and tents, outdoor banners, artistic fabric sculptures, liquid transportation containers, dry bags, and waterproof storage sacks. The products are generally lightweight, can be folded to a small size when not in use, have coatings that are generally impervious to industrial chemicals, and can be purchased in a wide range of colors, textures and fabric weights. Such fabrics generally come in approximately 5 to 12 foot wide rolls and thus must be cut into the required pattern pieces before being joined together to make the completed product.
There are three basic methods by which pieces of coated fabric can be joined together to create a mechanical bond as well as watertight and gas tight seams: traditional sewing followed by applying waterproof tape to the seam, gluing, and heat sealing (also called welding). Fabrics coated with certain rubber based coatings, such as Hypalon (manufactured by DuPont) can only be glued or sewn. Most of the newer coatings including polyurethane, polyvinylchloride, polypropylene, and polyethylene can either be glued or welded. However, gluing can be very labor intensive and further is subject to strict scrutiny from the Occupational Safety and Health Administration ("OSHA") due to the volatile solvents that are employed during the gluing operation. Most gluing now takes place in countries other than the United States.
There are four main methods of heat sealing or welding in use: hot wedge, radio frequency ("RF"), ultrasonic, and hot air. In the hot wedge method, two fabric pieces are drawn across a hot iron (or wedge) and then are pressed together. This method is quite similar to the hot air process with only the heat delivery system being different. One disadvantage of this method is that the wedge can become contaminated with dirt and melted plastic which then reduces the amount of heat delivered to the seam. Further, hot wedge welders typically experience a hot section immediately after the beginning of the weld as the wedge accumulates excess heat when idle. Another disadvantage is that, since the heat energy must pass through a solid object to reach the seam, the maximum speed of the hot wedge welding process is limited by the thermal conductivity of the wedge.
The RF method is probably the most widely used approach for heat sealing. The RF welder is basically an antenna (the die) that is poorly matched to the amplifier, thereby producing a great deal of heat rather than radio waves between the antenna and the underlying plate. In practice, the two fabric pieces are laid on the plate. The die is then brought down, thereby pressing the two pieces together. The operator then initiates the welding process by pressing a pair of push buttons. The actual RF process takes from about 5 to 15 seconds, depending upon the thickness of the pattern pieces and the amount of RF energy available from the machine.
There are, however, several disadvantages to the RF method. RF welding is a slow process because the size of the die is limited by the available energy of the unit. Typical RF welding dies are about 1 to 3 feet in length and approximately 1/2 inch wide. There is also some concern about the operator's health and safety as the operator is usually inches from an intense RF source which may be activated several hundred times in a typical shift. While RF health hazards have not been documented, it is known that stray RF energy from such machines can damage electrical equipment within approximately 50 feet of the machine and can light fluorescent fixtures located nearby. In addition, due to the die and plate arrangement, the RF method is typically limited to seams or joints that can be laid flat for welding. Three dimensional dies and plates are occasionally used, but are quite expensive and require a vacuum or other methods to hold the fabric in position as the die is applied. Further, the Federal Communications Commission ("FCC") has become increasingly strict regarding emissions of stray RF energy from industrial sources. Because of the increasingly strict FCC regulations, new RF welding equipment can typically cost $80,000 or more.
Ultrasonic welding is a process that is like RF welding, with the exception of the energy source. Rather than using radio waves, ultrasonic welding uses sound waves that basically vibrate the fabric molecules until sufficient heat is generated to melt the coatings.
In general, hot air welding is much faster than other methods, can accommodate three dimensional patterns, and requires no dies or tooling. In a hot air welder, the flow of hot air that floods the seam is not subject to contamination, as with the wedge welder, and there is no initial drop off of heat at the beginning of the seam. Most fabricators want the speed of hot air technology, but have felt that it is difficult to obtain consistent results for many types of coated fabrics and also that it requires highly trained operators.
The typical rotary hot air welding apparatus uses hot air to join together two pieces of plastic coated fabric. The welder first injects a stream of hot air from a hot air nozzle between the two pieces of coated fabric. The temperature of the hot air can be set in the range of approximately 500 to 1350 degrees F. The fabric pieces are then pinched between and pulled through the apparatus by two drive wheels. The distance from the hot air nozzle and the pinch point between the two wheels is in the range of approximately 0.5 to 0.75 inch. The wheel speed determines how long the fabric is exposed to the hot air stream before it passes between the wheels. With a constant air temperature, the amount of heat energy delivered to the fabric is inversely proportional to the wheel speed; a faster speed decreases the exposure and vice versa.
Commercial hot air welders currently available on the market have a number of shortcomings. One shortcoming is the lack of accurate control of the speed of the two drive wheels. If the wheel speed varies from the required speed, then the amount of heat delivered to the seam will vary. Too much heat supplied to the weld results in burnt fabric while too little heat results in cold welds or unwelded fabric.
The problem of providing accurate wheel speed is compounded by the need to control both wheels independently. For some fabric patterns, especially patterns with curves, one wheel may need to run slightly slower or faster than the other wheel. Commercial hot air welders typically use a single DC motor with a variable speed (voltage) amplifier. The drive energy from the motor passes through a long series of chains and pulleys to the bottom drive wheel. The drive energy to the top drive wheel first passes through a variable diameter pulley transmission that provides adjustment for the relative wheel speed and then passes through a similar set of chains and pulleys.
This arrangement is fairly inaccurate and is not easily or consistently repeatable. With any particular speed setting, the actual wheel speed can vary with both the temperature of the amplifier and the motor windings and with the load on the motor. This is typical of a DC drive system in which there is no feedback to the motor.
Further, the variable diameter pulley that provides differential speed control is an inherently inaccurate mechanical device. The same differential speed setting is not repeatable between consecutive seams. In view of the inaccurate DC drive system and the variable speed transmission for the drive wheels, wheel speed adjustment and calibration are constant problems. These problems are particularly evident when thinly coated fabrics are being welded and where the amount of heat energy delivered to the seam must lie within a narrow range.
In addition, the inaccurate control of the wheel speed results in the two edges of the fabric being joined not "in registration." In other words, at the end of the seam, one piece is shorter or longer than the other. Such an occurrence effects the overall quality of the product being made.
Thus, there is a need for a hot air welding apparatus that provides accurate control of the drive wheel speed and, consequently, accurate control of the amount of heat applied to the seam. There is a further need for a hot air welding apparatus that provides a differential speed setting that is repeatable between consecutive seams and that allows the drive wheel settings to be adjusted while the seam is being welded. In addition, there is a need for a hot air welding apparatus that joins the edges of two pieces of fabric "in registration."